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Data Flow Diagrams
Data Flow Diagrams
3
Data Flow Diagrams (DFD)
• DFDs describe the flow of data or
information into and out of a system
– what does the system do to the data?
• A DFD is a graphic representation of the
flow of data or information through a
system
4 Main Elements
• external entity - people or organisations that
send data into the system or receive data from
the system
• process - models what happens to the data i.e.
transforms incoming data into outgoing data
• data store - represents permanent data that is
used by the system
• data flow - models the actual flow of the data
between the other elements
Notation
Process box
D Data Store
External Entity
Data Flow
• Data Flow
• Process
• External Entity
• Data Store
4
Levelled DFDs
• Even a small system could have many
processes and data flows and DFD could
be large and messy
– use levelled DFDs - view system at different
levels of detail
– one overview and many progressively greater
detailed views
Level 0 - Context Diagram
• models system as one process box which represents scope of the system
• identifies external entities and related inputs and outputs
• Additional notation - system box
System boxExternal
entity
Data flow out
Data flow in
Level 1 - overview diagram
• gives overview of full system
• identifies major processes and data flows
between them
• identifies data stores that are used by the
major processes
• boundary of level 1 is the context diagram
Level 2 - detailed diagram
• level 1 process is expanded into more
detail
• each process in level 1 is decomposed to
show its constituent processes
• boundary of level 2 is the level 1 process
Other Notation
• Duplicates marked by diagonal line in
corner
• System Boundary
• Elementary Processes - star in corner
• Process that is levelled - dots on top
5
Rules for DFDs
• Numbering
• Labelling
• Balancing
Numbering
• On level 1 processes are numbered
1,2,3…
• On level 2 processes are numbered x.1,
x.2, x.3… where x is the number of the
parent level 1 process
• Number is used to uniquely identify
process not to represent any order of
processing
• Data store numbers usually D1, D2, D3...
Labelling
• Process label - short description of what
the process does, e.G. Price order
• Data flow label - noun representing the
data flowing through it e.G. Customer
payment
• Data store label - describes the type of
data stored
• Make labels as meaningful as possible
Balancing and data stores
• Balancing
– any data flows entering or leaving a parent
level must by equivalent to those on the child
level
• Data stores
– data stores that are local to a process need
not be included until the process is expanded
Data Flows
• Allowed to combine several data flows
from lower level diagrams at a higher level
under one data flow to reduce clutter
• Flows should be labelled except when
data to or from a data store consists of all
items in the data store
Joe’s Yard Joe’s builders’ suppliers has a shop and a yard. His system is entirely
manual. He has a stock list on the wall of his shop, complete with prices. When a builder wants to buy supplies, he goes into the shop and picks the stock items from the list. He writes his order on a duplicate docket and pays Joe, who stamps the docket as paid. The builder takes the duplicate docket and he goes to the yard and hands it to the yard foreman. The yard foreman gets the ordered items from the yard and gives them to the builder. The builder signs the duplicate docket and leaves one copy with the foreman and takes one copy as a receipt. Every week, Joe looks around the yard to see if any of his stock is running low. He rings up the relevant suppliers and reorders stock. He records the order in his order book, which is kept in the yard. The yard foreman takes delivery of the new stock and checks it against what has been ordered. He pays for it on delivery and staples the receipt into the order book. At the end of every month, Joe goes through all the dockets and the order book and produces a financial report for the shareholders.
Draw a context level DFD and a level-1 DFD for this system.
Context Diagram
• Find the people who send data into the system
– Often data is part of a PHYSICAL transaction
– When handing a bar of chocolate to a shopkeeper,
you are handing him/her a barcode.
• Find the people who get data out of the system.
– The only data you need is data that is transformed or
sent completely out of the system – not data that is
handled by an operator within the system.
Joe’s Yard Joe’s builders’ suppliers has a shop and a yard. His system is entirely
manual. He has a stock list on the wall of his shop, complete with prices. When a builder wants to buy supplies, he goes into the shop and picks the stock items from the list. He writes his order on a duplicate docket and pays Joe, who stamps the docket as paid. The builder takes the duplicate docket and he goes to the yard and hands it to the yard foreman. The yard foreman gets the ordered items from the yard and gives them to the builder. The builder signs the duplicate docket and leaves one copy with the foreman and takes one copy as a receipt. Every week, Joe looks around the yard to see if any of his stock is running low. He rings up the relevant suppliers and reorders stock. He records the order in his order book, which is kept in the yard. The yard foreman takes delivery of the new stock and checks it against what has been ordered. He pays for it on delivery and staples the receipt into the order book. At the end of every month, Joe goes through all the dockets and the order book and produces a financial report for the shareholders.
Draw a context level DFD and a level-1 DFD for this system.
Context diagram
Joe'sYard
Joe Customer
Supplier
Shareholders
Docket &Payment
Signed docket
Supply needs
Supply invoiceSupply order& payment
financialreport
Level-1 DFD processes Joe’s builders’ suppliers has a shop and a yard. His system is
entirely manual. He has a stock list on the wall of his shop, complete with prices. When a builder wants to buy supplies, he goes into the shop and picks the stock items from the list. He writes his order on a duplicate docket and pays Joe, who stamps the docket as paid. The builder takes the duplicate docket and he goes to the yard and hands it to the yard foreman. The yard foreman gets the ordered items from the yard and gives them to the builder. The builder signs the duplicate docket and leaves one copy with the foreman and takes one copy as a receipt. Every week, Joe looks around the yard to see if any of his stock is running low. He rings up the relevant suppliers and reorders stock. He records the order in his order book, which is kept in the yard. The yard foreman takes delivery of the new stock and checks it against what has been ordered. He pays for it on delivery and staples the receipt into the order book. At the end of every month, Joe goes through all the dockets and the order book and produces a financial report for the shareholders.
Verbs from script
• Has (passive)
• Buy supplies
• Picks stock items
• Writes order
• Pays joe
• Stamps docket
• Takes docket to yard
• Hands it to foreman
• Gets items
• Gives them to builder
• Builder signs docket
• Takes copy as receipt
• Looks around yard and reorders
• Records order in order book
• Foreman takes delivery – checks
• Foreman pays supplier
• Staples receipt to order book
• Produces financial report
Remove passive verbs and queries
• Passive: has stock list
• Buy supplies
– Picks stock items (views list)
– Writes orders
– Pays joe
– Stamps docket
• Customer then
– Takes docket to yard
– Hands it to foreman
• Gets items
– Gives them to builder
– Builder signs docket
– Takes copy as receipt
• Joe then
– Looks around yard and reorders
– Records order in order book
• Foreman
– takes delivery – checks
– Foreman pays supplier
– Staples receipt to order book
• Joe
– Produces financial report
Current Physical DFD
Current Physical DFD
Customer
Pickandpay
Shop1
*
Issuesupplies
Yard2
stock
DocketM1
Joe
*
Checkand
supply
Yard4
Orderbook
M3
Supplier
*
Acceptsupplies
Yard3
Supply receiptsM2
Pricelist
M4
*
Producefinancialreport
Joe's Office5Shareholders
choice
Duplicatedocket
required stockrequired stock
docket
Duplicate dock-et (receipt)
Supply needs
Supply needs
supplies
Supply invoice
Supply order
Supply receipts
Supply payment
Payment
choice
Price
Supply receipts
docket
financialreport
Supply needs
supplies
Level 1 current physical
Customer
Buysupplies
20
getitems
12
Reordersupplies
21
Restock
22
*
Producefinancialreport
Joe's Office5
Joe
Orderbook
M3
DocketM1
Money
stock
Shareholders
Supplier
Foreman
*
Taketo
yard
11Supply needs
financialreport
Docket &Payment
Supply order
Docket
Signed docket
Signed docket
Signed docket
Payment
required stock
required stock
completeddocket copy
completeddocket copy
Supply order
supplies
supplies
Payment Payment
Supplierreceipt
Supplierreceipt
completeddocket copy
buildersignature
Buy Supplies
Buy supplies20 DocketM1
Money
Customer
*
WritesOrder
20.1
*
PaysJoe
20.2*
StampDocket(signatu-
re)
20.3
Docket
Payment
Docket
PaymentDocket
Get Items
get items12
Foreman
stock
*
Give itemsto customer
12.1
*
Get buildersignature
12.2
*
Givecopy
as receipt
12.3
Customer
Signed docket
required stock
buildersignature
Signed docket
completeddocket copy completed
docket copy
Reorder supplies
Reorder supplies21
Orderbook
M3
Joe
*
Reorderfromyard
21.1
*
Recordorder
21.2
Supply order
Supply needs
Supply needs
Restock
Restock22
stock
Orderbook
M3
Supplier
Money
*
Takedelivery
22.1
*
Paysupplier
22.2
supplies
Payment
Supplierreceipt
Supply order
supplies
Payment
Supply order
